1. Field of the Invention
The present invention relates to an optical near-field generating element, which is preferably used for an optical head or a probe array head of an optical memory, a fiber probe for a microscope, an exposing head in a fine machining apparatus containing an exposing apparatus and the like, and various optical apparatuses including them.
2. Description of the Related Art
The light emitted from an optical head used for an optical memory, such as a recordable optical disc and the like, needs to be basically fine focused in order to make a record density higher. For this reason, conventionally, instead of red laser light, blue laser light having a shorter wavelength is used, and a focus performance in a lens, such as a solid immersion lens and the like, is improved. Typically, since the light is diffracted, it is said that the treatment of a substance smaller than the wavelength of the light is substantially impossible in principle. The minimum dimension that can be treated depending on this light diffraction is referred to as a diffraction limit. In view of this diffraction limit, it is already difficult to make the record density dramatically higher in the optical memory. Similarly, in view of the diffraction limit, even in a case of an optical microscope, it is already difficult to improve a resolution in an optical system using a usual lens. In a case of an optically fine machining apparatus, an optical communication system, an optical device or various other optical apparatus, it is already difficult to attain a dramatically high performance. Thus, it is requested to develop a technique of substantially fine focusing light based on another principle without the reduction in the wavelength and the improvement of the lens performance.
Under such a request, a research has been advanced with regard to an optical microscope and an optical head of an optical memory using an optical near-field generating element that can be used as light equivalent to a very fine focused light. Already, it has been put to practical use with regard to an optical microscope of a type in which the optical near-field generating element is generated at the tip of a fiber probe.
Here, Optical Near-Field will be explained in brief. A hole having a diameter shorter than a wavelength of light is made on a light shielding film. The light is emitted to the hole from one side (input side) of the light shielding film. At this time, the light neither diffuses nor passes through the hole. However, a surface wave of the light is confined near the hole, and a thin layer of the light having spherical surface is formed near the hole on the other side(output side) of the light shielding film. This surface wave of the light is referred to as the optical near-field. For example, in a case of light having a wavelength of 400 nm, a surface wave, which is exponentially attenuated, collectively exists within a sphere having a radius of about 400 nm, for a hole having a diameter of 400 nm or less. Also, in a case of a diameter of 100 nm, the surface wave, which is exponentially attenuated, collectively exists within a sphere having a radius of about 100 nm. Such a surface wave is never outputted or propagated from the output side under that condition. However, under the condition that this optical near-field is generated, for example, if a surface of an optically recording medium as an optical memory or a surface of an inspection target. sample of a microscope approaches until it comes in contact with a thin layer of lights constituting the surface wave serving as the optical near-field, namely, if it approaches to a situation within several hundred nm to several ten nm, the surface wave confined in this extremely small region is outputted from the output side of the hole towards the approaching surface. Thus, the optical near-field can be used as the light equivalent to the very fine focused light. Moreover, it is expected to be applied to a higher density recording, a higher resolution, a super fine machining process in a next generation of an optical apparatus.
However, if the optical near-field is generated, the strength of the light that can be used as the optical near-field on the output side is extremely low as compared with the strength or the amount of the incident light. For example, if the amount of the light that can be used as the optical near-field is converted into a transmission rate implying a transmission through the hole, it is only about several 1/1000% to several 1/100%. Moreover, this transmission rate becomes further reduced as the optical near-field is focused smaller by making the hole smaller. Thus, as the optical near-field is focused smaller in order to make the density of the optical memory higher, the amount of the usable light is reduced. After all, this results in a problem that its practical usage is essentially difficult.
In particular, in a case of a service such as a microscope in which a speed is not important, the practical usage can be attained even if a weak optical near-field is ineffectively used. However, in a case of a service such as an optical memory requiring a writing speed and a reading speed, a service such as an optical communication system or an optical device requiring an operational speed, and a service such as a fine processing apparatus requiring a machining speed such as an exposing speed and the like, even if the light can be focused smaller, if its optical strength is extremely reduced, there is almost no utility worth on the practical usage. Hence, this problem is extremely severe on the actual usage.